The invention relates to an apparatus for producing a plasma and for the treatment of substrates therein, with a microwave generator, a chamber to contain a gas, a magnet system for producing local electron-cyclotron resonances, and with a substrate for coating in the chamber.
In numerous fields of technology it is necessary to apply very thin coatings of pure substances to certain objects. An example is window glass which is provided with a thin coating of metal or metal oxide in order to filter certain wavelength ranges out of sunlight. In semiconductor technology, thin coatings of one or more substances are often applied to a substrate. It is especially important that the thin coatings not only be pure, but also that they be precisely measured out so that the coating thicknesses--and, in the case of coatings of chemical compounds, their composition--will be accurately repeatable. These coating thicknesses are, as a rule, between two and several thousands of nanometers.
A variety of methods are known for applying thin coatings to films, glass and other substrates. In a first method, the thin coating is applied by chemical or electrochemical deposition, while in a second method the coating is applied by evaporation in a vacuum. With evaporation it is difficult to provide large areas with very thin coatings with the required uniform precision and repeatability, and consequently a third method, known as the sputtering or cathode spraying process, is used. For the deposition of a thin coating from the gas phase, sputtering is, of course, unsuitable.
To be able to deposit a pure substance or a chemical compound from the gaseous phase, the substance or compound is converted to the plasma state. The radicals formed in the plasma deposit themselves on the substrate. For the production of such a plasma, different forms of electrical energy can serve. For example it is possible to use direct currents, low-frequency alternating currents or corona discharges for the production of plasmas. Especially advantageous is the production of plasma by microwaves, because in this case no electrodes are needed, which can contaminate and become ablated, and because the plasma produced by microwaves has a greater density of ions and radicals and therefore can be kept at a higher pressure than the plasma produced by other methods. Furthermore, the chemical structure of starting monomers can be preserved at least partially. Lastly, the microwave plasma is also favored for the establishment of cold cathode ion sources.
It is true that usually only small volumes of plasma can be produced by microwaves, because the apparatus by which the microwave energy is delivered to the plasma--e.g., antennas, waveguides and cavity resonators--do not permit the production of large volumes of plasma. To produce a gas plasma, the delivered electrical field strength must exceed the electrical breakdown field strength of the gas. Since the breakdown field strength increases with increasing the pressure, high electrical field strengths are necessary at high pressures.
An apparatus for the production of plasmas by means of electromagnetic radiation is known, with which high field strengths are produced (U.S. Pat. No. 3,814,983).
In this apparatus a delay line, i.e., a microwave conductor of low group velocity ("slow wave structure") is used for the purpose of feeding the electrical energy to the plasma, the energy source being located outside of the receptacle and its electrical field passing through the receptacle wall. This delay line consists of a "semiradiating" system about 90 cm long, which operates in the degenerate .pi./2 mode or close to the degenerate .pi./2 mode. Operation in the vicinity of the band edge, i.e., either in the degenerate .pi./2 mode or in the .pi. mode, leads to especially strong electrical fields in the vicinity of the delay line. The reason for this lies in the circumstance that the electrical field strength is inversely proportional to the group velocity of the wave, which in the vicinity of the edge of the band assumes a very small value. Furthermore, in this system the electrical field strength decreases with the distance perpendicular to the plane of the delay line. It is true that with this apparatus no large-volume plasmas with a very large, uniform plasma zone can be produced. It follows that the rate of deposit of polymers is irregular across the entire substrate width in the known apparatus. Moreover, interactions take place between the waves, which occur in the delay line, in the window dielectric and in the plasma; i.e., poorly understood interferences develop, which adversely effect the configuration of the plasma zone.
To equalize the rate of deposition in the case of polymers it has already been proposed, in an apparatus according to U.S. Pat. No. 3,814,983, that, in addition to the known delay line, at least a second elongated delay line be disposed on the same side of the substrate (German Federal Pat. 31 47 986). But this "crossed structure" arrangement has the disadvantage that the strongest plasma burns directly at the inside of the microwave window where the microwave is injected, and this results in an especially great and undesirable coating of this window.
Furthermore, an apparatus is known whereby a plasma is produced by means of a high-frequency wave which is injected into a waveguide in which a glass tube is situated in which the plasma is produced (German Federal OS 31 44 016), to which U.S. Pat. No. 4,438,368 corresponds. Around the plasma producing tube there is in this case provided a coil which produces a magnetic field along the axis of the glass tube. At a circuit frequency .omega. of the high-frequency field, and a magnetic flux density B, the electron-cyclotron resonance frequency will be .omega.=e .times.B/m. At this resonance frequency the coupling of the high-frequency wave to the plasma electrons is especially strong. It is a disadvantage even in this known device, however, that only relatively small plasma zones can be produced. Furthermore, the glass tube easily takes on coatings deposited from the gas phase.
A microwave plasma source is also known, which has a vacuum chamber that serves as the discharge chamber (U.S. Pat. No. 4,433,228). The microwave energy in this case is fed into the discharge chamber through a microwave propagation path.
Outside of the discharge chamber and the microwave propagation path permanent magnets are provided, which serve for the guidance of the plasma produced by the microwave. The magnetic fields of these permanent magnets do not, however, permit cyclotron resonance of the plasma electrons in a defined area of a treatment chamber.
Another known microwave plasma source is largely the same as the plasma source according to U.S. Pat. No. 4,438,368, but an additional magnet coil is provided behind the substrate that is to be treated (Kimura, Murakami, Miyake, Warabisako, Sunami and Tokuyama: "Low Temperature Oxidation of Silicon in a Microwave-Discharged Oxygen Plasma", J. Electrochem. Soc., Solid-State Science and Technology, Vol. 132, No. 6, 1985, pp. 1460 -1466, FIG. 1). An especially interesting application for these known plasma sources might be, for example, the coating of searchlight reflectors with aluminum and a plasma-polymerized protective coating. Heretofore this coating has been performed in so-called batch coaters, using a direct-current plasma, a hydrophilization of the surface being performed in some cases by the addition of oxygen.
Also known is the depositing of silane and N.sub.2 O for the purpose of producing SiO.sub.2 coatings containing hydrogen. In this case high-frequency plasmas are used, as a rule (cf. D. P. Hess: J. Vac. Sci. Technol. A, 2, 1984, 244). To optimize the quality of the deposited film in the broadest sense, however, very high flows of N.sub.2 O are required in proportion to silane, for example of 20 : 1 to 100 : 1 (cf. E. P. G. T van de Ven, Solid State Technol. 24, 1981, 167). Typical deposition rates range around 10 mm/min.
Apparatus is provided whereby it will be possible on the one hand to produce a uniform, large-volume plasma, and on the other hand to keep the plasma away from the microwave window.
According to the inventive process, it is possible to provide a transparent coating of SiO.sub.x, where 1 &lt;.times.&lt;2, to a substrate, especially a surface coated with aluminum. This is accomplished by introducing a hydrogen silicide gas into a chamber, as well as a second reactive gas consisting of oxygen or an oxygen containing compound. The chamber is exposed to microwaves and a magnetic field of sufficient strength to form a plasma of both gases in a region thereof. A substrate in said region is thus coated with SiO.sub.x, x being determined by the ratio of gases admitted.
The advantage achieved with the invention consists especially in the fact that large-area, uniform plasmas can be produced. Another advantage is that no deposits form on the entry window. These advantages are due to the fact that the magnetic field produced by the magnet systems is strong enough, at least in some areas, to permit a so-called electron-cyclotron resonance. Use is made of the fact that the electrical field strength that is necessary for the ignition of the plasma in a region in which the electron cyclotron resonance can take place is considerably smaller than in a region free of a magnetic field. Through the localization of the magnetic field sufficient for the electron cyclotron resonance, it is thus possible also to produce a corresponding localization of the plasma production. Furthermore, the apparatus according to the invention is especially suitable for the coating of substrates moving in a continuous linear manner.